Typically, external disk drive products are either configured as Direct Attached Storage (DAS) or Network Attached Storage (NAS) drives. DAS drives can only be used with one computer at a time and are connected using a Universal Serial Bus (USB), Thunderbolt, 1394, eSATA, or other type of cable. NAS drives have an Ethernet or wireless LAN (Wi-Fi) interface and can be accessed by multiple computers simultaneously.
DAS devices are generally simpler in design than NAS drives, in that DAS drives need only allow the host (e.g., a PC) to read/write data sectors on its hard disk drive (HDD); they do not parse the file system that the host creates and do not manage the user's files. Because DAS drives operate at the sector level, hosts can create any number of partitions and file systems on the drive. A NAS drive, in contrast, is essentially a small server: it can run a mainstream operating system (OS) such as a customized version of Linux or Microsoft Windows, for example, and provides file sharing services and other features. NAS devices manage their HDDs the same way a full-featured personal computer (PC) or server does, as they partitions their drive(s) according to their requirements and do not allow the user to create arbitrary partitions.